This invention relates to a method for producing a carbonated beverage can. More particularly, it relates to a method for producing a thin wall thermoplastic carbonated beverage can based on a polyolefin which has acceptable yield strength, modulus, and creep resistance values.
U.S. Pat. No. 4,379,014 discloses a method for the manufacture of packing containers. A container body is manufactured by spiral winding of the monoaxially molecular-oriented polyester film. A polyester strip which is monoaxially molecular oriented in the strip direction is coated with a layer of non-molecular oriented amorphous polyester material. The coating layers may be polyethylene as is disclosed at Column 2, lines 60-63. However, as the specification of U.S. Pat. No. 4,379,014 states in Column 2, lines 63-67, sometimes the polyethylene materials do not give good sealant strength or the overlapped joint that the material requires is too wide. This results in a lack of good adhesion and as a result, the coating material may need to be a modified polyester (PETG). The polyester strip of U.S. Pat. No. 4,379,014 is wrapped around the can body in only one direction.
U.S. Pat. No. 4,181,239 discloses a cylindrical container body for packaging pressurized carbonated beverages, however, it requires at least two layers of a polyester film.
U.S. Pat. No. 4,451,524 discloses a polypropylene strap for use in packaging articles. The polypropylene strap does not require subjection to a surface embossing process to prevent it from being longitudinally split and in addition has a tensile strength higher than that of conventional polypropylene strap and good elongation resistance, high rigidity and is flexible. This patent does not address the problems inherent in a pressurized container, such as a carbonated beverage container, such as the necessity for sufficient volume swell resistance in both hoop and axial directions.
Copending application, K-4466, Ser. No. 646,387 was filed Aug. 31, 1984, and discloses a method of manufacturing a pressure pipe by mixing a blend of polypropylene and polybutylene, extruding the blend into film, stretching the film and heating to a temperature between the two melting points of the polypropylene and the polybutylene, forming said film into tapes, and forming the tape into pipe lengths. However, the pipe produced according to K-4466 does not disclose the high modulus values and acceptable creep resistance and yield strength values of applicant's invention. Applicant's invention is not a blend and need not be heated to a temperature between the melting point of polypropylene and polybutylene. In actuality, a blend might weaken the carbonated beverage can of applicant's invention.
Numerous factors govern the requirements for a carbonated beverage container. For example, in soft drink beverages, (1) container vendability in coin operated machines is a key requirement. A 12 ounce can must fit into and dispense from the many existing vending machines. The overall outside dimensions of any 12 ounce can are constrained, limited and fixed to those ordinarily found for the aluminum and steel cans for which the vending machines have been designed to accept and to vend. Also, the beverage capacity in the can--12 ounces--is fixed. Therefore, the can diameter, can height and can wall thickness must be designed in concert to satisfy 12 ounce can vendability requirements.
Another key factor is (2) consistency of dimensions under conditions of storage, be it in a warehouse, on a delivery truck or on a supermarket shelf. A carbonated beverage can is a pressure vessel. Typical soft drink beverages will have an internal pressuire of about 55 psig once the beverage is filled with four volumes of CO.sub.2 gas, sealed with an end or lid and then brought to room temperature. With temperatures rising much above 105.degree. F., internal pressures of about 87 psig are achieved as the CO.sub.2 gas expands and exerts a higher internal pressure as it warms. This is the pressure at which a conventional easy open metal end on a metal can will buckle. Below this temperature and pressure, the can sidewall material must not appreciably creep, deform or grow in size to an unacceptable level. The can body must not split, twist, torque or otherwise fail up to this lid buckling pressure of 87 psig. It is known that for a cylindrical shell or a cylindrical can body, hoop direction pressure induced stress in the can body sidewall is twice as large as the axial direction pressure induced stress in the can body sidewall. With metal cans for carbonated beverages the can body material is necessarily homogeneous and monolayer, i.e. it consists of a uniform gauge and thickness material of singular components, e.g. thin aluminum or thin steel sidewall material. In such monolayer, homogeneous material containers, including homogeneous, monolayer plastic containers, the can sidewall must necessarily be engineered so that the more severe design criterion--the hoop stress--is satisfied. Therefore, monolayer, homogeneous can bodies have been limited to always satisfying the more severe hoop stress requirement while being over engineered (by default) in satisfying the less demanding criterion, the axial stress requirement.
A container for which the can body sidewall materials and method of layered construction provide a disproportionation of stress resisting properties such that the 2:1 hoop stress to axial stress forces in a cylindrical can body geometry are better satisfied, has been long needed in the industry.
Can performance, that is, satisfactory sidewall resistance to the stresses created by the internal pressure of the carbonation, is largely governed by key can wall material properties; namely, (1) elastic modulus, (2) initial tensile yield strength; and (3) creep resistance is also a significant factor. Ultimate breaking strength or ultimate breaking elongation are of little consequence. The elastic modulus of the sidewall material must be sufficiently high so that deformation, i.e. volume swell of the container, is minimized. The initial tensile yield strength must be sufficiently high so that the sidewall stresses, (these stresses are a function of sidewall thickness, pressurization level and can diameter (for hoop stress)) are resisted and little strain (deformation) occurs. The properties of high modulus and high yield strength in combination are the key to satisfying the thinnest wall possible, especially when the construction is achieved as taught in our invention to disproportionate these properties to satisfy the unbalanced hoop and axial stresses in the can sidewall.
One necessity for thin wall has been recited. Other crucial factors, such as end and top seaming of end closures, faster beverage cooling, minimization of the number of layers to be wrapped to facilitate manufacturing, the lessening of the number of adhesive layers necessary to bond wound layers in the construction, and lighter weight depend upon achieving the thinnest wall possible as taught in our invention.
It has been discovered that a polyolefin, and most particularly, polypropylene can be used to manufacture a thin wall carbonated beverage can, and that the use of a polyolefin or polypropylene, when oriented to a degree which assures high modulus and high initial tensile yield strength (for example, when compared to the modulus and initial tensile yield strength values for PET polyester or oriented PET), allows for thin-walled carbonated beverage containers. This results in less use of thermoplastic materials in making the can and thus greater efficiency in its manufacture. By "thin-wall", we mean a wall thickness of less than 80 mils, preferably less than 50 mils, and most preferably less than 30 mils.
Heretofore, unoriented polypropylene films and sheet have been used for various other applications but have low modulus, low initial tensile yield strength and poor creep. As such, these films and sheet are unacceptable to make thin-walled carbonated beverage cans as modulus values of the unoriented films are from 2.5 to 5 times too low, and initial tensile yield values are about 3 to 4 times too low. Furthermore, high quality biaxially oriented polypropylene film, for example Hercules T-503 film, while having a high modulus, shows unacceptable initial tensile yield values, about 2 times too low. Monoaxially oriented polypropylene manufactured for strapping tape shows good modulus and initial tensile yield strength, but is much too thick to achieve a thin wall carbonated beverage can construction. Further, strapping tape is only available in relatively narrow widths.
Two proprietary technologies have been developed in oriented polypropylene film or sheet materials that overcome these past deficiencies, i.e. offer a combination of high modulus, high tensile yield strength with improved creep properties, and also have film or sheet widths which are satisfactory to practice the instant invention. Further, through these development efforts in this field, film or sheet material of oriented polypropylene with properties superior to oriented PET film or oriented PET blown carbonated beverage bottle materials have been developed. Prior to these developments, thin wall polypropylene carbonated beverage cans were not possible. Heretofore, many different composite cans have been made, such as cans for motor oil or food products and the like. However, these composite cans are not manufactured to withstand internal pressure as found in a carbonated beverage can.
We have discovered that cross plying of strength film materials at plus/minus angles to one another not only circumvents these deficiencies found in the past containers and methods, but also affords a method by which the hoop and axial stresses can be accommodated at the 2:1 relative ratio, making possible a thin-walled carbonated beverage can with oriented polyolefin or polypropylene wrapped film layers, with the oriented polyolefin or polypropylene showing high modulus and high tensile yield strength.
It has been discovered that highly oriented polyolefins can be used to manufacture a carbonated beverage can and that the use of the polyolefins results in acceptable modulus, creep and yield strength values and better modulus and yield strength values than commercially available PET or other polyester carbonated beverage containers. The higher the modulus, the thinner the can wall can be made for end seaming. This results in less use of thermoplastic materials in making the can and thus greater efficiency in the manufacture of the can.
The higher the modulus, the thinner the can wall that can be made and, thus, the more easily an aluminum end could be attached to the can body. It is necessary that a carbonated beverage can wall be sufficiently thin since a standard aluminum end or top may be attached to no more than about 30 mils thickness of can wall.
We have discovered that unidirectional wrapping leads to undesirable torquing and twisting of the can body in response to the unbalanced hoop and axial stresses in the can with pressurization, and that overcoming these unbalanced stresses by wrapping sufficient extra layers of material in one direction preclude the achievement of thin wall body material necessary for end attachment, etc. Clearly, such torquing and twisting is unacceptable from a graphics/aesthetic point of view for a decorated can.